1. Field of the Invention
The present invention relates to a acrylic glass tinted with organic pigments, with improved resistance to weathering, and a process for preparing such a acrylic glass.
2. Discussion of the Background
The basic materials for different types of acrylic glass, especially poly(methyl methacrylate), but in many instances also copolymers of methyl methacrylate, represent a practically colorless substance, whose excellent resistance to light and weathering is retained even when tinted, and which can be used for long-term applications, for example as a plate. Plastics, including acrylic plastics and, in particular, types of acrylic glass based on poly(methyl methacrylate), are used in the tinted state, to a great extent. The colorants for tinting plastics must fulfill a number of technical requirements including heat resistance, oxidation resistance, migration resistance, light resistance, resistance to weathering, and dispersibility during processing.
Transparent tints are mainly achieved with organic dyes which are truly or colloidally soluble in the monomer or the polymer, while translucent and opaque tints are obtained by using inorganic white and colored pigments or metal powder (Kunststoff-Handbuch, Volume IX, Polymethacrylate, Carl Hanser Verlag, Munich, 1975).
For tinting acrylic glass, cadmium pigments were or are used to a significant extent, particularly the composition cadmium sulfide or cadmium sulfide/selenide, with a color range from greenish yellow via red to dark wine red. Along with excellent dying properties and good light and heat resistance, these pigments are also characterized by good resistance to weathering. Because of their low solubility, cadmium pigments are considered to be essentially non-toxic. However, cadmium is released in an environmentally harmful form in the disposal of wastes containing cadmium, especially if they are incinerated.
For the production of acrylic glass which is practically identical in color with those tinted with cadmium pigments, but free of cadmium pigments, organic coloring pigments available on the market are being tested for their utility and are being used. Instead of organic coloring pigments, colorants which consist of a soluble organic dye and pigment, e.g., titanium dioxide, i.e., pigment preparations, can also be tested and used as a cadmium pigment substitute. (Ullmann, Encyklopadie der technischen Chemie, 4th edition, Volume 15, pp. 275-279).
Previous work with substitute tints similar to cadmium pigment has shown that acrylic glass tinted in this way has significantly reduced resistance to weathering as compared with known types of acrylic glass tinted with cadmium pigments.
Thus, the acrylic glass produced with organic coloring pigments or with organic dyes cannot meet all the requirements as they are known in the state of the art and demanded for acrylic glass which contains inorganic colored pigments. In addition to poor distribution of the colorants in the glass, the long-term behavior of the tints in acrylic glass, in particular, shows that their resistance to weathering must be improved. Thus, there remains a need for acrylic glass tinted with an organic pigment or an organic dye which is free of the above-described drawbacks.